Alkaline earth metal hydrocarbyl sulfonates have long been used as detergents in crankcase lubricating oils. They are often overbased to provide an alkaline reserve in the oil which neutralizes acids and inhibits corrosion. Overbasing methods are disclosed in U.S. Pat. No. 2,956,018 and U.S. Pat. No. 3,027,325.
Promoters have been used in the overbasing stage such as the ammonium salt of aliphatic acids described in U.S. Pat. No. 3,609,076.
The overbasing converts excess alkaline earth metal base in the mixture to colloidal metal carbonate which is stabilized in the oil diluent by alkaline earth metal sulfonate detergent (referred to as "soap") in the oil. It is most desirable to have as high a content of colloidal alkaline earth metal carbonate as can be stabilized in the oil. It is also desirable from an economic viewpoint to use as little soap as possible in stabilizing the colloidal alkaline earth metal carbonate. Such a product having maximum alkaline earth metal carbonate and minimum alkaline earth metal sulfonate is referred to as a "high-base:low-soap" product. When making high-base:low-soap calcium additives it has in the past been customary to conduct the overbasing reaction of carbon dioxide with alkaline earth metal base in two stages introducing only part of the base in each stage. If it was attempted to add the entire amount of metal base and to overbase in one stage the reaction mixture would become cloudy and gel, giving an unacceptable product.